File associate by MIME subtype / extension

I'm writing files in markdown format using the markdown editor Remarkable and I name these files with the extension *.md. I'd like to associate the these files with Remarkable but if I do so the result is that any plain text file text/plain is associated with Remarkable which is not what I want.

If I issue metype myTextFile.md I get text/markdown so I would have expected that I could associate by the subtype.
How can this be achieved?